This invention relates to an electronic ballast circuit for operation of an electric discharge lamp, and more particularly to a miniature electronic ballast that produces a relatively small amount of radio frequency interference (RFI).
It is now common practice to operate many types of discharge lamps at a high frequency in order to achieve the numerous advantages of such high frequency operation. Therefore, the operating frequencies of dc-to-dc converters have been increased to the MHz range, resulting in a significant reduction of both size and weight of the overall power supply arrangement. This is due to the fact that the size and weight of a magnetic component is inversely proportional to its operating frequency and the magnetic components are the bulky space consuming components in any power supply. Therefore, an electronic ballast can be miniaturized if its operating frequency is increased because the bulky components in an electronic ballast are similarly the magnetic components such as inductors and transformers.
In principle, therefore, it should be possible to reduce or miniaturize the size of an electronic ballast circuit for discharge lamps by increasing the operating frequency of the system. However, in the case where the frequency of the lamp current and voltage is the same as the operating frequency of the electronic ballast, the problem of radio frequency interference (RFI) arises, especially if the operating frequency is increased to approximately 100 KHz or higher. At these frequencies, the influence of parasitic parameters of outgoing wires will have a considerable effect, which will degrade the performance of an electronic ballast. In addition, the discharge lamp itself will radiate energy and thus will also provide significant radio frequency interference.
One prior art circuit for operating a low-pressure mercury discharge lamp with a high-frequency pulsatory current is shown in European Patent Application 0 439 861 A1. This patent application describes an electronic ballast circuit for operating a low pressure mercury discharge lamp with a sinusoidal amplitude modulated high frequency current. A carrier frequency of approximately 50 KHz is amplitude modulated and the high frequency amplitude modulated current is applied directly to the discharge lamp. The amplitude modulation in this patent is employed in order to keep the light color of the low pressure mercury discharge lamp from shifting due to dimming of the lamp because of adjustment of the amplitude of the amplitude modulated lamp current.
U.S. Pat. No. 5,142,201 (Aug. 25, 1992) describes a circuit including a dc/ac converter for supplying power to a high frequency electrodeless discharge lamp from a dc voltage source. This type of lamp is designed to operate at high frequencies, usually in the megahertz (MHz) range. This circuit does not use amplitude modulation since it is not required for the electrodeless lamp. However, operation of the lamp at such high frequencies will cause severe electromagnetic interference (EMI) and radio frequency interference problems. Since the electrodeless lamp has to be operated at high frequency (MHz range) in order to generate light, expensive EMI and RFI suppression circuits have to be employed so as to reduce the amount of EMI and RFI to acceptable levels.